Flexible kesterite thin-film solar cells under stress

Abstract

Understanding the stress-induced physical phenomena is essential for improving the long-term application of flexible solar cells to non-flat surfaces on buildings and in mobile utilities. Here, we investigated the electronic band structure and carrier transport mechanism of Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic devices under mechanical stress. We fabricated highly efficient flexible CZTSSe devices on a Mo metal foil substrate by controlling the incorporation of Na. When mechanical stress was applied to the CZTSSe material, electronic structure of CZTSSe was deformed as the band gap, valence band edge, and work function increased or decreased. Additionally, electrical properties of bent CZTSSe surface were probed by Kelvin prove force microscopy and the CZTSSe with Na doping showed less degraded carrier transport in the bending state (either concave or convex) compared to the CZTSSe without Na. Furthermore, the local open-circuit voltage (VOC) on the CZTSSe surface decreased under mechanical stress due to limited carrier excitation. The reduction of local VOC with mechanical bending occurred larger with convex bending than in concave bending, which is consistent with the degradation of device parameters with bending. This study paves the way for understanding the stress-induced optoelectronic changes in flexible photovoltaic devices beyond the kesterite-based devices.